As Paul stood in the middle of the legislative body, he began with an ambiguous "compliment." He observed that they were "very religious" in all aspects of their lives (i.e., they were respectful of the divine). This is a good characteristic to have, as long as one's religion is pleasing to God. Those who are only religious when it is convenient are hypocrites. The Athenians were very religious, though they have some serious misunderstandings that Paul will address. Pliny recorded that in the days of Nero, Athens was adorned with some 30,000 public idols, and that didn't even include the ones in private homes! This caused some in the past to sarcastically affirm that it was easier to find a god in Athens than a man!
The audience Paul addressed here was likely exclusively Gentile. Thus, with no Jews or proselytes present, it would do him little good to begin by speaking of the long-expected Messiah (of which these people were ignorant). Before he can preach Jesus the Christ to them, he must first correct their flawed conception of deity. Anytime a speaker addresses an audience, he must consider their level of knowledge and begin there. Paul did such masterfully, and it is for this reason that his recorded speech here is so much different than the other ones Luke preserved via inspiration of the Holy Spirit.
Paul had seen an altar for THE UNKNOWN GOD. Why would there be such an altar to begin with? Because the Athenians were "very religious"! They didn't want to mistakenly leave any divine being out. It is ironic, of course, that the only true and living God is the unknown divine being to the Athenians. They ignorantly worshiped so many false gods and the true God without knowing Him. This would not do, and Paul intends to educate those who will hear him out.
"[The] God, who made the world and everything in it, since He is Lord of heaven and earth, does not dwell in temples made with hands. Nor is He worshiped with men's hands, as though He needed anything, since He gives to all life, breath, and all things. And He has made from one blood every nation of men to dwell on all the face of the earth, and has determined their preappointed times and the boundaries of the dwellings, so that they should seek the Lord, in the hope that they might grope for Him and find Him" (Acts 17:24-27).
The God who was unknown to the Athenians was the true and living God. He was known by Paul as the One who had created everything! This true God is living and the giver of life. But, as an eternal spiritual being, He does not live in physical structures made by mortals. He is too big and powerful for that! He is Lord (i.e., Master or Ruler) and thus exercises authority over His creation. Furthermore, the true God does not need anything from man, like food offerings, for example. However, He does desire faithful obedience since it is the best thing for us (wherein we will find real purpose and contentment). He is the Sustainer, not the sustained!
Paul then states that all humans share a common ancestry (and it is not through primates). Genesis teaches that we all descend ultimately from Adam and Eve, and more specifically, from Noah and his wife. As a side note, those who have studied genetics have shown that there is no genetic difficulty in having the wide array of diversity we see today in the human population (regarding skin color and other unique traits) all coming from a single couple. The interested student is encouraged to visit www.ApologeticsPress.org and search for "Origin of Races" for more information.
Not only has the true and living God created mankind, He has also set certain boundaries on mortals, which He regulates providentially to the fulfillment of His will. To say that God determines "preappointed times and the boundaries of their dwellings" is, in essence, to say that God--not blind fate--rules in the kingdoms of men (cf. Dan. 4:25)! God has these parameters in place so mankind would realize His sovereignty and consequently seek Him! That's what God wants; He wants us to search for Him and, by virtue of revelation, find Him! Humans have both the need and ability to seek God, and they are under obligation to do so. Paul is trying to help the Athenians do that very thing.